It is known to use a broadband radio frequency (RF) front end to capture a broad band of RF signals, convert this band of RF signals down to a band of intermediate frequency (IF) signals, and then select the desired signal from this band of IF signals by tuning into the IF frequency which represents the desired signal.
It has also been suggested to use switched capacitor networks to form an IF bandpass filter to eliminate signals outside of the desired band of IF signals.
It is also known to heterodyne a signal, such as an IF signal, directly to baseband to recover the communicated information, that is, the modulating signal. However, heterodyning a signal directly to baseband may cause a loss of phase (I and Q) information. Thus, in some cases, it is preferable that the signal be converted to a band which is lower in frequency than the original signal, but not converted to baseband.
To capture an entire band of signals, such as the conventional consumer FM broadcast band of 88 to 108 MHz, an IF bandwidth of 20 MHz is needed, so the IF center frequency must be above 10 MHz. For example, if an IF frequency of 25 MHz is used then the IF amplifier must have a bandpass of 15 MHz to 35 MHz. However, analog IF networks which are capable of operation at such IF frequencies and bandwidths are generally expensive, noisy, and/or lossy.
In addition, the automatic gain control (AGC) circuit must have a bandwidth similar to the IF bandwidth, which also raises the issues of cost, noise, and loss with respect to the AGC circuitry. In addition, the broadband conversion process from the RF band to the IF band results in image or adjacent signals, which may have frequencies near the desired signal frequency, and which may have a signal strength comparable to, or even greater than, the desired signal. A strong image or adjacent signal can thus cause the AGC circuit to detect a strong signal and reduce the gain accordingly, even though the strong signal is the wrong signal. These processes have the effect of raising the noise figure of the receiver, thus pushing the desired signal closer to, or sometimes even down into, the noise floor.
A switched capacitor network can be integrated onto or as part of an integrated circuit (IC), thus saving space. In addition, a switched capacitor network generally costs less than a typical IF filter and provides equivalent rejection characteristics. Further, a switched capacitor network is often less lossy than other types of IF filters, thus reducing the number of IF amplifiers. Thus, the prior art discloses several uses for, and benefits of, switched capacitor networks.
However, the prior art does not disclose or suggest that a switched capacitor network can be used to perform both tuning and down-conversion of a signal.
Further, the prior art does not disclose or suggest how an AGC control can be generated which is not responsive to the signal strength of an alias, an adjacent channel, or a spurious conversion product.